Infusion devices may be used to deliver an infusion media (e.g. a medication such as insulin) to a patient. Such devices may be designed to be implanted into a patient's body to deliver predetermined dosages of the infusion media to a particular location within the patient's body; e.g. in the venous system, the spinal column, or within the peritoneal cavity.
A known infusion device of the type described above includes a drive mechanism that includes a reciprocating pumping element made of a ferrous material. The reciprocating pumping element includes an actuator including a piston portion that is coupled to an armature portion. The piston portion is configured to reciprocate within a piston channel when a solenoid coil is alternately energized and de-energized. That is, when the solenoid is energized, magnetic flux causes the actuator to move very quickly (i.e. in the order of 2-3 milliseconds) until it reaches a stop member. This corresponds to the pump's forward stroke and results in the delivery of a predetermined dosage of infusion media from an outlet chamber to the patient. When the solenoid is de-energized, the lack of magnetic flux allows the actuator to return to its original position under the force of a spring. This, in turn, causes the pressure in the piston chamber to fall. The reduced pressure in the piston chamber causes infusion media to flow from a reservoir through an annulus between the actuator piston and the piston cylinder wall to refill the piston chamber thus equalizing the pressure between the reservoir and the piston chamber and preparing the pump for its next pumping or delivery stroke. This is referred to as the refill stroke. The annulus between the actuator piston and the piston cylinder is very small (i.e. in the order of 150 to 250 microinches radially) resulting in an outlet chamber refill process that takes between about 1 to 2 seconds. In contrast, the pump's forward (delivery) stroke may be approximately 500 times faster than the refill process.
Over time, protein drugs such as insulin denature resulting in the deposition of protein on the surfaces of fluid paths; for example, on the surfaces that form the annulus between the actuator piston and the pistol cylinder. Such deposits may cause valves to leak, impede the motion of moving parts, and/or otherwise degrade device performance. Typically, such deposits are removed periodically (e.g. once per year) by rinsing the implanted pump with a solvent (for example, sodium hydroxide (NaOH)) causing the deposits to dissolve.
The rinsing procedure is typically performed as follows. The infusion device's reservoir is first filled with a desired buffer or rinsing solution. Since the device is implanted near the patient's skin, the reservoir may be filled utilizing a first syringe. A second syringe engages the device's outlet to produce a negative pressure differential and therefore help pull the fluid through the pump. The pump itself is operated during this procedure to assist fluid flow through the pump. In the case of insulin, it is an established goal that the rinsing procedure should result in the transport of at least 1 cc of rinsing fluid from the inlet reservoir to the pump's outlet in approximately ten minutes. Rinse cycles less than ten minutes in duration may result in failure to dissolve all deposits, and rinse cycles greater than ten minutes may result in undue discomfort to the patient. The rinse procedure may include a multi-stage operation that involves emptying and refilling the pump's reservoir several times with different fluids, and different drug therapies may require the use of different rinsing agents. It is to be understood that other protein drugs may require different rinse times and/or volumes.
As previously stated, the space or annulus between the surface of the actuator piston and the piston cylinder wall is approximately 150-200 micro-inches radially, a fairly tight fit, and it takes approximately 1 to 2 seconds to refill the piston chamber via this annulus. Deposits of the type described above that form on the annulus walls will restrict fluid flow thus increasing the time it takes to refill the piston chamber, which, in turn, lowers the stroking frequency and causes the corrective rinse procedure to be protracted; e.g. it could take 30 minutes or more instead of the desired 10 minutes. The deposit build-up could be so extreme so as to cause the pump to jam. In this case, it could take more than 30 minutes to pass ¼-½ cc of rinsing fluid. This may not be sufficient to render the pump operational.